1. Field of the Invention
The present invention relates to force-balance accelerometers and, more particularly, to feedback loops for the pick-off coils of force-balance accelerators.
2. Description of the Related Art
In U.S. Pat. No. 4,088,027, issued May 9, 1978 to Hernandez et al., there is disclosed a force balance servo accelerometer including a D'Arsonval type mechanism for rebalancing, between a pair of sensing coils, a seismic mass moved by acceleration. The D'Arsonval type mechanism comprises a pair of suspension beams mounted in parallel planes in a liquid filled cylindrical housing. A pair of axially aligned taut wires support a torque coil between the suspension beams. The coil surrounds a permanent magnet fixedly mounted in the housing. An arm extending outwardly from the coil, transverse to the axis of the taut wires, supports the seismic mass between the sensing coils, which are mounted in the housing. The sensing coils form two arms of a bridge circuit energized by an oscillator connected across one pair of opposing terminals of the bridge. The signal developed across the other pair of opposing terminals is applied to a differential amplifier. The resultant difference signal is sine wave multiplied with the output of the oscillator in a quadrature detector. The output of the quadrature detector, which is related to the acceleration of the seismic mass, is applied to the coil of the D'Arsonval mechanism to rebalance the seismic mass. A bellows mounted on one end of the housing allows the liquid in the housing to expand and contract as the temperature of the environment in which the D'Arsonval type mechanism is located changes.
In U.S. Pat. No. 4,315,434, issued Feb. 16, 1982 to Marcus R. Eastman, there is disclosed an accelerometer, the output of which is sent to a pulse width modulating digitizing circuitry comprising a comparator to generate a PWM signal, a flip-flop steering circuit, an “H” switch to toggle a torquer constant current either positive or negative, and an AND gate to gate clock pulses for the output.
The accelerometer taught in the afore-said U.S. Pat. No. 4,315,434 has the disadvantage of a continuous torquer current, i.e. a bipolar current through a torque coil, and therefore a large power consumption. Furthermore, with reference to FIG. 2 of Eastman, it is clear that the feedback loop from a pick-off coil (14) to a torque coil (24) consists entirely of analog components, with exception of the flip-flop (22). Therefore, the accelerometer of the Eastman reference is very susceptible to aging and drift of components, temperature, and noise sources. The Eastman reference does not provide a technique to compensate for the above effects on the components, and therefore must be returned to the manufacturer for calibration.
An additional disadvantage of that accelerometer is that it does not provide a measured value for the acceleration, but instead, provides a digital pulse train that is proportional to the sensed acceleration (see column 2, line 6). Clearly, additional circuitry is required in order to obtain a measurement of the acceleration.
That prior accelerometer has the disadvantage of including much of the feedback loop of the accelerometer in the analog domain, which makes it overly sensitive to temperature, manufacturing and aging tolerances.